1. Field of the Invention
The invention concerns aluminum oxide masses with very narrow pore radius distribution which are particularly suited for production of catalysts and/or adsorption agents.
2. Description of Related Art
It is normally desirable that catalysts have a large surface. Since the surface can be increased only slowly and expensively by size reduction, an attempt is made to give a catalyst material a large "internal surface" by formation of very fine channels, so-called pores. The following relation applies to cylindrical pores: EQU Internal surface=2.times.pore volume/pore radius.
For pores having a radius of 4 nm and a pore volume of 1 cm.sup.3 g.sup.-1, an internal surface area of 1000 m.sup.2 g.sup.-1. is obtained. Such materials are already commercially available.
However, the size of the internal surface area is not the only thing that matters, but also that the walls of the catalyst pores have the correct appropriate structure. Wall structures of catalyst pores (acid, basic centers, openings, metal clusters, etc.) are considered the cause for active sites.
The pores of now common catalysts, except for the special case of zeolites, are not uniform in pore size and pore shape. Pores with more than 25 nm radius are referred to as macropores, pores with a radius from 1 to 25 nm are called mesopores. Pores with a radius smaller than 1 nm are the so-called micropores.
The macro- and mesopores are generally caused by the spacing between primary particles. The micropores are attributed to cracks with almost atomic dimensions within the primary particles. In the zeolites, which occur in nature, but can also be synthesized, cuboctahedral structures are present. Their structural elements are Si--O--Al structures. Their peculiarities consist of the fact that they surround relatively large cavities (cages) that are accessible via comparatively narrow openings. They have a very large internal surface area in the form of their pores with fixed symmetry.